1. Field of the Invention
The present invention generally relates to an optoelectric apparatus for, and a method of, detecting the presence or absence of a marking on articles of manufacture such as, for example, commutation tickets, telephone cards, prepaid cards or any other items desired to be detected, verified or identified for any particular purpose. The present invention is particularly applicable where the marking on the articles of manufacture is formed by the use of a printing or inking medium of a kind containing at least one fluorescent substance.
2. Description of the Background Art
Japanese Laid-open Patent Publication No. 5-20512, published in 1993, discloses an optoelectric apparatus for and a method of detecting the site at which a marking is formed by detecting the presence or absence of a fluorescent signal emitted from the marking. More specifically, according to this publication, the marking containing at least one fluorescent substance is first illuminated intermittently to repeatedly excite the fluorescent substance to thereby emit fluorescent light which is subsequently detected during a non-illuminating period. The detected fluorescent light is an indication of the site at which the marking is formed, or the presence of the marking at such particular site.
With the above-described prior art method utilizing the persistent fluorescent signal, it has, however, been found that the fluorescent detecting apparatus tends to be fooled by the presence of an external light of a wavelength matching with or in the vicinity of the wavelength of the fluorescent signal. This is particularly true where the marking is illuminated under an environment in which such an external light is persistent. This results in an erroneous indication. Therefore, in order to avoid the erroneous indication, the conventional fluorescent detecting apparatus requires at least its optical detecting system to be shielded from the external light and is thus limited in application.
The inventors of the present invention have conducted a series of investigations in an attempt to substantially eliminate the inconveniences inherent in the conventional apparatus. In the course of the investigations, the inventors have found that while the relationship between the irradiating light and the resultant fluorescent signal varies in unison with a relationship substantially equivalent to the relation between the voltage, applied to a CR (capacitor and resistor) circuit, and the voltage across the capacitor of the CR circuit, the external disturbing light generally has an intensity which does not vary substantially or fluctuates inconsistently and that utilization of the phase relationship among these signals makes it possible to accurately extract a fluorescent light component out from various light components.
In other words, when the marking containing the fluorescent substance is irradiated by light having its amplitude varying cyclically, the light emanating from the marking contains a background light component reflected from the marking and its surroundings, a fluorescent light component of a frequency equal to that of the background light component, but retarded 45.degree. in phase from that of the background light component, and an external disturbing light component. Accordingly, if the frequency component that is equal to the frequency of the irradiating light is selectively extracted, the fluorescent signal can be isolated from the other light components by the utilization of the difference in phase therebetween.
It has also been found that, considering that the difference in phase between the background light component and the fluorescent signal results from the difference in type of the fluorescent substance used, accurate information on the marking can be obtained if the difference in phase is discriminated. Speaking differently, although formation of the marking of an identical shape with the use of an arbitrarily chosen fluorescent substance should pose no difficulty to any person, it is physically difficult to use a fluorescent substance exactly identical in composition with that used in the marking and, therefore, utilization of the marking containing a particular fluorescent substance as a security marking which may include a marking invisible to the human eyes is effective to avoid the opportunity of making a forgery.